The present invention relates to a directly imageable planographic printing plate produced by using laser beam for direct plate making. In particular, this invention serves effectively to produce a directly imageable waterless planographic printing plate that makes it possible to perform printing without dampening water.
The so-called directly imageable plate making, which produces an offset plate directly from a block copy without the need of using film for plate making, has many good features such as simplicity to eliminate the need of expertise, rapidity to produce a printing plate quickly, and rationality to permit selection of a desirable system from a wide range of printers depending on the required quality and available cost. With such good features, this method has been in wider use in the fields of general offset printing and flexographic printing as well as small offset printing. Recently, in particular, various new directly imageable planographic printing plates have been developed in response to the rapidly increasing availability of improved output systems including pre-press systems, image setters, and laser beam printers. Directly imageable planographic printing plates can be divided into some groups in terms of the mechanism used for plate making. Thus, they may use laser beam irradiation, thermal heads for imaging, pin electrodes for partial application of voltage, or ink-jet printing to form an ink-repelling layer or an ink-holding layer. Among others, the use of laser beam permits an enhanced definition and increased plate making speed, resulting in the availability of a variety of products of this type.
Laser-imaging type printing plates are further divided into two groups: photon type that use a photoreaction and thermal type that use photothermal conversion to cause a thermal reaction. The thermal type products, in particular, are currently becoming the mainstream as a result of rapid improvement of semiconductor laser generators to be used as light source.
Different methods have been proposed to provide thermal-type planographic printing plates, particularly for waterless printing which does not need dampening water to be fed during the printing process. However, they all have different problems.
Some patents such as U.S. Pat. Nos. 5,339,737, 5,353,705 and EP0580393 have disclosed inventions relating to methods for producing a printing plate based on abrasion of a thermo-sensitive layer. Although they are advantageous in terms of simple developing operations, they need high laser output for the abrasion of the thermo-sensitive layer, and gas and other by-products of abrasion can have adverse effect on the optical system. Furthermore, additional operations are required for removing abrasion residue, and it is difficult to reproduce fine halftone dots.
A new type of printing plates in which the thermo-sensitive layer remain have been developed, as proposed in EP0914942 for example. Such inventions have provided directly imageable waterless planographic printing plates that serve for the production of high quality prints, which is one of the most important features of waterless printing, without having adverse effect on the optical system. These printing plates, however, contain a thermo-sensitive layer remaining in both the laser-irradiated and non-irradiated portions, leading to problems with difficult plate checking due to low contrast between the laser-irradiated and non-irradiated portions when observed visually. Thus there are difficulties in performing checking during the process after the preparation of a printing plate until the start of printing, and in allowing a plate mounted on the press to be checked during printing. Such checking is intended to confirm, prior to or during printing, that the printing plate has been prepared as required. Visual observation is performed for this confirmation. By this plate checking, mistakes associated with data transfer from computers to the process machine and mistakes during the plate making can be eliminated to prevent inferior prints from being produced due to such mistakes.
The present invention comprises the following.
A method of producing a directly imageable planographic printing plate that comprises;
applying laser beam to a directly imageable planographic printing plate precursor comprising at least a substrate, a thermo-sensitive layer and a ink-repelling layer in this order,
removing the ink-repelling layer from the laser-irradiated portion, and dyeing, with a dyeing solution, of the printing area which is free of said ink-repelling layer,
wherein the difference between the reflected absorption of the non-printing area which holds the ink-repelling layer and the reflected absorption of said printing area, observed after dyeing at the absorption maximum wavelength of the dye in said dyeing solution, is not less than 0.3 and not more than 2.0.
A directly imageable planographic printing plate precursor comprising at least a substrate, a thermo-sensitive layer and a ink-repelling layer in this order, wherein the ultraviolet absorption spectrum observed by the reflection method has the following features:
(1) the main peak in the ultraviolet absorption spectrum exists between 700 nm and 1,200 nm, and
(2) the ratio A/B of the absorbance at 830 nm (A) to the absorbance at 650 nm (B) is not less than 1.5.
A directly imageable planographic printing plate precursor comprising at least a substrate, a thermo-sensitive layer and a ink-repelling layer in this order, wherein the ultraviolet absorption spectrum observed by the transmission method has the following features:
(1) the main peak in the ultraviolet absorption spectrum exists between 700 nm and 1,200 nm, and
(2) the ratio A/B of the absorbance at 830 nm (A) and the absorbance at 650 nm (B) is not less than 3.0.
A directly imageable planographic printing plate comprising at least a substrate, a thermo-sensitive layer and a ink-repelling layer in this order, wherein the ultraviolet absorption spectrum observed by the reflection method has the following features:
(1) the main peak in the ultraviolet absorption spectrum exists between 700 nm and 1,200 nm, and
(2) the ratio A/B of the absorbance at 830 nm (A) to the absorbance at 650 nm (B) is not less than 1.5.
A directly imageable planographic printing plate comprising at least a substrate, a thermo-sensitive layer and a ink-repelling layer in this order, wherein the ultraviolet absorption spectrum observed by the transmission method has the following features:
(1) the main peak in the ultraviolet absorption spectrum exists between 700 nm and 1,200 nm, and
(2) the ratio A/B of the absorbance at 830 nm (A) to the absorbance at 650 nm (B) is not less than 3.0.
A directly imageable planographic printing plate comprising at least a substrate, a thermo-sensitive layer and a ink-repelling layer in this order, wherein the thermo-sensitive layer in the printing area contains a dye that has an absorption maximum in the range of 400 nm to 700 nm, and the difference between the reflected absorption of the non-printing area and the reflected absorption of the printing area, observed at the absorption maximum wavelength of the dye, is not less than 0.3 and not more than 2.0.
The present invention serves to produce a directly imageable planographic printing plate that has such good features as high image reproductivity and easy plate checking operation.